1. Field Of The Invention
This invention relates generally to bleomycins, and more particularly to bleomycin analogs, useful as radiopharmaceuticals, which have a metal-chelating substituent covalently bound to and modifying a bleomycin.
The invention described herein was made in the course of, or under, a grant from the National Institutes of Health.
2. Description Of The Prior Art
Bleomycins are anti-tumor antibiotics discovered by Hamao Umezawa, et al and reported in Journal of Anti-Biotics 19A, page 200 (1966).
Commercially available bleomycin is a mixture of bleomycins which differ only in a terminal amine. For example, the terminal amine of bleomycin A.sub.2 is a 3-dimethyl-sulfopropylamino moiety whereas the terminal amine of bleomycin B.sub.2 is a 4-guanidinobutylamino moiety. However, as used hereinafter, the general term "bleomycin" shall include mixtures of bleomycins; whereas when reference to a specific type, differing at the terminal amine, is preferred, such type shall be indicated.
Bleomycin's affinity for tumorous tissue, coupled with its ability to complex with a variety of metal ions, has aroused considerable interest in its potential as a tumor visualizing agent. The rationale behind this has been that the bleomycin complex of a gamma-ray emitting metal ion would localize in tumor tissue in vivo; the size and location of malignant tissue could then be determined for diagnostic purposes. .sup.57 Co-bleomycin was the first bleomycin complex to be used clinically as a tumor locating agent. Because of the 270 day radioactive half life of .sup.57 Co, several other bleomycin-metal complexes have since been investigated.
.sup.67 Ga, .sup.59 Fe and .sup.62 Zn are all poorly complexed by bleomycin. Labeling of bleomycin with .sup.59 Fe(III) has been attempted; however, Fe(III) is readily hydrolyzed in neutral aqueous solution and determined not to be bound to bleomycin. Ga(III)-bleomycin has been determined to be unstable in neutral and alkaline solutions. In vivo studies in tumor bearing mice have also demonstrated the instability of the .sup.67 Ga(III)-bleomycin complex. In a comparative study of the .sup.57 Co, .sup.62 Zn and .sup.111 In complexes of bleomycin, it has been found that the tissue distribution of .sup.62 Zn following injection of the .sup.62 Zn-bleomycin complex mimicked that of an injection of .sup.62 ZnCl.sub.2, which suggests that the .sup.62 Zn-bleomycin complex dissociates in vivo.
Mixed reports have been given concerning the ability of bleomycin to complex with .sup.99m Tc, as formation of the .sup.99m Tc-bleomycin complex is very dependent upon the pH and concentration of reducing agent present. It is unlikely that the .sup.99m Tc-bleomycin chelate remains intact in vivo as .sup.99m Tc appears to be associated with human serum albumin 4 hours after injection of .sup.99m Tc-bleomycin.
The bleomycin chelate of .sup.111 In(III) has received a great deal of attention. However, it has been shown that .sup.111 In(III) does not remain bound to bleomycin in vivo. Within four hours after injection of .sup.111 In-bleomycin, .sup.111 In has been found bound to serum transferrin in human subjects.
Encouraging clinical results have been obtained with the .sup.57 Co-bleomycin complex. The distinguishing feature of Co-bleomycin, as opposed to the other metal ion-bleomycin complexes hereto known is the stability of the complex. Although the complex is prepared by combining CoCl.sub.2 with bleomycin, chelated Co(II) may be air oxidized to give Co(III). Complexes of the latter are inert to ligand exchange. Co-bleomycin has been shown to be inert and stable in vivo.
Thus, of all the metal ion-bleomycin chelates which have been known, only that of .sup.57 Co has been found to remain intact in vivo. Apparently as a result of this stability, .sup.57 Co-bleomycin has given excellent tumor images. While its chemical inertness and in vivo behaviour would appear to make .sup.57 Co-bleomycin an ideal compound for tumor imaging, .sup.57 Co has the undesirable physical property of a 270 day radioactive half-life. Ideally, a radionuclide used for diagnostic purposes should have a half-life of several hours to a few days and should emit only gamma radiation, with energy between 100 and 400 KeV. The longer lived radionuclides, such as .sup.57 Co, pose serious contamination and health problems. Unfortunately, none of the other isotopes of cobalt have the desired physical properties.